1. Field of the Invention
The present invention relates to an inverter driving circuit for an inverter circuit including a plurality of pairs of switching elements for converting a direct current to an alternating current, and more particularly to a system of gate driving circuits for the switching elements.
2. Description of the Prior Art
Inverter circuits for converting a direct current to an alternating current by using a plurality of switching elements, which are utilized to drive an induction motor, a brushless motor or an uninterruptive power supply, are well known. A typical inverter circuit is shown, for example, in FIG. 12. In FIG. 12, a plurality (three in this example) of pairs of switching elements (Q), which are coupled in series to each other, are coupled to DC power source 1 in parallel to each other. Referring to FIGS. 13 and 14, gate driving circuits GD1--GD3 and GD1'--GD3' are coupled to switching elements Q1--Q6, respectively. Switching control circuit 3 is coupled to the respective gate driving circuits GD1--GD3 and GD1'--GD3+. Over current detecting circuit OC is coupled to switching control circuit 3. Output terminals of three-phase current (U, V, W) are connected to a three-phase load 2 such as a three-phase motor.
In such a conventional inverter circuit, gate driving circuits GD1--GD3 and GD1'--GD3' for switching elements Q1--Q6 all use a photocoupler (the details of this type gate driving circuit are described later with reference to FIG. 3) or a pulse transformer (the details of this type gate driving circuit are described later with reference to FIG. 2).
Conventional gate driving circuits for inverter circuits have a number of disadvantages. For example, a gate driving circuit such as shown in FIG. 3, which uses a photocoupler 4, requires two power sources V1 and V2. If such a gate driving circuit is employed for each of switching elements Q1-Q6, six power sources are required for switching elements Q1-Q3 coupled to the positive side of DC power source 1 and two power sources are required for switching elements Q4-Q6 coupled to the negative side of DC power source 1 (because a common emitter is used on this side). Consequently, the total number of power sources required is eight, which is an unacceptably high number for many reasons. On the other hand, a gate driving circuit such as shown in FIG. 2, which uses a pulse transformer 13, requires no additional power source. However, if this type of gate driving circuit is employed for each of switching elements Q1-Q6, the following disadvantages occur.
The latter type of gate driving circuit has poor switching response. In particular, the delay time in transmission of an off signal is relatively long. For example, in the circuit shown in FIG. 12, if a short occurs in load 2 (for example, a motor), an over current is detected by over current detecting circuit OC. Switching control circuit 3, which is responsive to over current detecting circuit OC, then turns off all switching elements Q1-Q6. However, because there is a relatively long delay time in transmission of an off signal, the switching elements often cannot be protected quickly enough.
Namely, there is a risk that one or more of the switching elements may be shorted.
Furthermore, the switching loss in the latter type of gate driving circuit (that is, a circuit using a pulse transformer) is large as compared with that in the gate driving circuit shown in FIG. 3 (that is, a circuit using a photocoupler), because the switching speed thereof is faster than that of the gate driving circuit shown in FIG. 3. Therefore, when the control signal sent to the gate driving circuit is chopped to change the rotational speed of a motor provided as load 2, if the chopping is conducted at a high frequency, the inverter efficiency is greatly reduced.